Alloys of the Fe-Co type are known as soft magnetic materials having a maximum saturated magnetic flux density amongst all magnetic materials. They are expected to have utility for motors and magnetic yokes which are required to transmit high magnetic energy in spite of small dimensions. Fe-Co type alloys in the form of ingots are however accompanied by the drawback that they do not practically permit cold working because of their brittleness.
It has hence been attempted to improve the cold workability by adding vanadium. No sufficient cold workability has however been achieved yet, although some improvements are observed.
Powder metallurgy are considered to be a useful means for overcoming such poor workability. This process however has difficulties in achieving densification of sintered products, so that materials having practical magnetic properties have not been obtained. A variety of methods have hence been proposed.
For example, it is attempted in Japanese Patent Application Laid-Open No. 291934/1986 to improve the compressibility and sinterability by using an Fe-Co alloy in which no ordered lattices have been formed. In Japanese Patent Application Laid-Open No. 54041/1987, the sintered density has been improved by hot isostatic press (HIP) processing. In Japanese Patent Application Laid-Open No. 142750/1987, the green density and sintered density have been improved by using a coarse Fe-Co alloy powder and a fine Co powder in combination.
However, all of these proposals use compression forming. They can use only poor-sinterability coarse powders of such a size that the compressibility is not impaired and the powders are not taken into mold clearances. The resulting sintered materials therefore have low magnetic properties. There has thus been a demand for sintered materials having still higher magnetic properties.
Further, Japanese Patent Application Laid-Open No. 85650/1980 discloses the attempted production of a high-density sintered material by adding 0.1-0.4% of boron to an alloy of the Fe-Co type.
On the other hand, Japanese Patent Publication No. 38663/1982 (Japanese Patent Application Laid-Open No. 85649/1980) discloses the attempted production of a high-density sintered material by adding 0.05-0.7% of phosphorus to an alloy of the Fe-Co type.
However, all of these methods enhances densification by using the formation of a transitional liquid phase in the course of sintering, which in turn relies upon a third element. It is thus necessary to strictly control the sintering temperature within a narrow range, thereby making it difficult to achieve a high yield upon mass production. Moreover, the elements whose addition is proposed are considered to aggravate the brittleness of Fe-Co alloys, leading to the problem that cracking or chipping may take place in a working step in which sintered products are finished into precision parts.
In addition, Japanese Patent Application Laid-Open Nos. 291934/1986 and 142750/1987 require a sintering treatment at a temperature as high as 1300.degree.-1400.degree. C., while Japanese Patent Application Laid-Open No. 54041/1987 needs a high pressure of at least 800 atm in addition to the sintering at a high temperature of about 1300.degree. C. It is hence not only difficult to conduct mass production but also necessary to use special facilities. The methods of these publications are therefore not economical.
On the other hand, materials consisting practically of Fe and Co alone have a low electrical resistivity and their core loss values increase when employed under ac power. It may hence be contemplated to add a third component to a material of the Fe-Co type. For example, materials of the Fe-Co-V type exhibit improved ac properties. However, such a third component involves a problem that it is prone to oxidation upon sintering. This approach therefore has the problem of inferior dc properties as long as a production process capable of inhibiting oxidation is not developed.
An object of this invention is to provide a sintered Fe-Co type magnetic material which can be worked into intricate shapes, has excellent dc magnetic properties, a low core loss and a high saturated magnetic flux density, and also to provide its production process excellent in economy.
Another object of this invention is to provide a sintered Fe-Co type magnetic material having a small core loss value when employed under ac power and superb ac magnetic properties, and also to provide a production process thereof, said process featuring easy molding and the possibility of elimination of C, derived from an organic binder, without extreme oxidation of its components.